Marine reservoir ages for coastal West Africa

Soulet, Guillaume; Maestrati, Philippe; Gofas, Serge; Bayon, Germain; Dewilde, Fabien; Labonne, Maylis; Dennielou, Bernard; Ferraton, Franck; Siani, Giuseppe

doi:https://doi.org/10.5194/gchron-5-345-2023

Articles | Volume 5, issue 2

https://doi.org/10.5194/gchron-5-345-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gchron-5-345-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 5, issue 2

Research article

|

17 Aug 2023

Research article |

| 17 Aug 2023

Marine reservoir ages for coastal West Africa

Guillaume Soulet, Philippe Maestrati, Serge Gofas, Germain Bayon, Fabien Dewilde, Maylis Labonne, Bernard Dennielou, Franck Ferraton, and Giuseppe Siani

Download

Final revised paper (published on 17 Aug 2023)
Supplement to the final revised paper
Preprint (discussion started on 21 Mar 2023)
Supplement to the preprint

Interactive discussion

Status: closed

CC1:
'Comment on gchron-2023-5', Paula Reimer, 08 Apr 2023

The presentation of marine reservoir ages for the west coast of Africa by Soulet et al. provides much needed data for calibrating radiocarbon ages of carbonates from marine organisms for this region. While the main results are not unexpected, they will give confidence to archaeologists and geoscientists using radiocarbon dates of shells or foraminifera for age models. It is interesting that two shells from near the discharge of the nutrient-rich Ogooué and Congo Rivers had lower reservoir ages than nearby shells but having been collected in the 1930s and 1940s fit the global decline in R. It would be worth mentioning that this global decline is due to fossil fuel input to the atmosphere (e.g. Druffel & Suess 1983). The manuscript is well-written and contains important details on the samples used that are often omitted in publications. I would definitely recommend publishing this paper with minor corrections.
Specific comments.
Line 41: ‘Larger ΔR values are located at high-latitudes’. I would suggest qualifying this as ‘Most larger ΔR values’ as there are low-latitude locations where ΔR values are higher due to groundwater carbonates being leached into coastal water (e.g. Hadden & Cherkinsky 2015) or where upwelling increases the ΔR values (e.g. Gulf of California, Goodfriend & Flessa 1997).
Line 501-504: ‘The fact that species living in very different ecological habitats (e.g., Senilia senilis in lagoons/semi-enclosed bays and Donax rugosus on beaches exposed to heavy surf; see also section material) show similar reservoir age values (R or ΔR) suggests that the habitat only exerts a minor influence on measured reservoir age.’ There are many examples where habitat does exert a major influence on R so please clarify that this conclusion is for the regions studied.
Line 561-562: ‘It is thus possible that these outlier samples were transported subfossil samples that died a century or more before collection date’ Two of these outlier samples were listed as having ligaments attached so it seems very unlikely that they had been dead for very long before collection.
Figure 1 caption: please specify what PG, LO, PN and Ca stand for on the map.

There are also some grammatical errors that need to be corrected by a fluent English speaker. Also the word ‘specie’ is not correct as ‘species’ is both singular and plural.

DRUFFEL, E. M. & SUESS, H. E. 1983. On the Radiocarbon Record in Banded Corals - Exchange Parameters and Net Transport of (Co2)-C-14 between Atmosphere and Surface Ocean. Journal of Geophysical Research-Oceans and Atmospheres, 88, 1271-1280.
GOODFRIEND, G. A. & FLESSA, K. W. 1997. Radiocarbon reservoir ages in the Gulf of California: Roles of upwelling and flow from the Colorado River. Radiocarbon, 39, 139-148.
HADDEN, C. S. & CHERKINSKY, A. 2015. 14C variations in pre-bomb nearshore habitats of the Florida Panhandle, USA. Radiocarbon, 57, 469-479.

Citation: https://doi.org/10.5194/gchron-2023-5-CC1
- AC1: 'Reply on CC1', Guillaume Soulet, 05 Jun 2023
  
  We thank Paula Reimer very much for her comments. Please see our response in the attached pdf files.
  
  Citation: https://doi.org/10.5194/gchron-2023-5-AC1
RC1:
'Review: No C cycle role in MRA around 1900 AD', Anonymous Referee #1, 12 Apr 2023

See PDF attached.

Citation: https://doi.org/10.5194/gchron-2023-5-RC1
- AC2: 'Reply on RC1', Guillaume Soulet, 05 Jun 2023
  
  We thank the Anonymous Reviewer for his comments. Please, see our response in the attached pdf file.
  
  Citation: https://doi.org/10.5194/gchron-2023-5-AC2
RC2:
'Comment on gchron-2023-5', Michel Fontugne, 24 Apr 2023

gchron-2023-5 Marine reservoir ages for coastal West Africa by Soulet & al.
The article by Soulet & al presents the 14C reservoir age measurements of seawater for the western coast of Africa, from Morocco to Angola. This type of data is lacking in the community and therefore deserves to be published. The calculation of reservoir ages and ∆R does not require any additional comment. Nevertheless, some points remain obscure and therefore call for further clarification Sampling. As for the results published by Ndeye (2008), a large number of samples come from the MNHN Paris collections and more particularly from the Gruvel missions. Soulet & al. classify five results as aberrant, in particular those from Guinea and the Ivory Coast which were collected by dredging (see Dautzenberg, Annales Inst. Océanogr.). This method of collection is not the most appropriate for this kind of study…. Ndeye had obtained 6 aberrant results including 4 for samples from the MNHN. The sample from Jacqueville and Ile de Roume come from the shell of B. ringens, which could encourage us to requalify this species for this type of study.
Outlying samples.
The other so-called aberrant samples are collected on coasts subject to the influence of deep water rises (upwelling) whose CID is depleted in 14C. The values obtained from Morocco to Dakar are compatible or characteristic of these upwelling zones. For comparison, the authors can refer to the reservoir ages calculated for the Peruvian upwelling (Kennett & al, 2002; Fontugne & al, 2004; Jones & al 2007, 2010; Owen, 2002, in Radiocarbon Ortlieb & al QR 2011; Etayo -Cadavid et al Geology 2013). Surprisingly, these references are absent when the authors mention the zones of deep water upwelling (see in Outlier specimens).The works cited above demonstrate the extreme variability of the values of the reservoir ages depending on the position of the upwelling cells, the intensity of the winds but also a variability during the period of life of the mollusk. The sample from Lake Ahémé (Benin) with a ∂13C of -4.76‰ seems to have been marked by a strong contribution of carbon of continental origin likely to reduce the reservoir age value. As this lake is located 10km from the coast with narrow communication with the coastal lagoons, it cannot really be considered as representative of the Atlantic Ocean. It would be better not to take it into account but the result can be published in the table S1.This sample comes from the Gruvel mission. Given the number of aberrant samples provided by this mission to the authors as well as to Ndeye (2008), it would be desirable to verify the harvesting conditions for all these samples. References. Not all bibliographic references are adequate: Stuiver et al 1986, Stuiver & Brasiunas 1993 (both in Radiocarbon) would be more justified for the definitions of R and ∆R (as mentioned later in the text). Stuiver and Pollack (1977) only specify that reservoir age corrections should not enter into the calculation of conventional 14C dates. As with ocean/atmosphere CO₂ exchanges, it would be more accurate to mention the pioneering works of Revelle & Suess, 1957 and Craig, 1957 (both in Tellus). Brocker & al., 1985 and Stuiver 1980 (both in JGR) could also have been mentioned; these studies giving the distribution of 14C in the ocean and highlight the equatorial upwelling (or divergence) characterized by low 14C water. Of course, Bard et al 1988 can be added.The reference to the Black Sea, which is a lake occasionally connected to the Mediterranean Sea, is not the most judicious. A reference to the Baltic Sea (see Lougheed & al Clim. Past 2013) would reinforce this point. These remarks are not exhaustive, I think that the authors will be able to complete the bibliographical references with more precision. -Stable Carbon Isotopes and reservoir ages uncertainties. The authors indicate reproducibilities of ±0.04‰ and ±0.02‰ for the values of δ18O and δ13C, respectively. These are occasional measurements integrating a seasonal variability of 0.5 to 1‰ recorded by the growth rings of the shell (see among others Carré & al, 3P 2005, Jones & al RC, 2007, 2010. Jones' work also shows a seasonal variation of about 100 years in marine reservoir age. Jones' work also shows a seasonal variation of about 100 years in marine reservoir age. Perhaps, the authors could comment on the evolution of reservoir ages between 1890 & 1950 by reconsidering this new level of uncertainty In detail Table S1Columns F and G should simply be labeled Latitude and Longitude, the minus sign indicating S and W respectively.The authors wrote “Upwelled waters can be depleted in 14C relative to the sea surface” ; but upwelled water are always depleted in 14C. The upwellings areas are the atmosphere’s largest natural source of CO2 (see among others Takahashi & al Deep Sea Res., 2002).

To conclude, this paper needs more precisions. Nevertheless, its data are needed.

Citation: https://doi.org/10.5194/gchron-2023-5-RC2
- AC3: 'Reply on RC2', Guillaume Soulet, 05 Jun 2023
  
  We thnak Michel Fontugne very much for his comments. Please, see our response in the attached pdf file.
  
  Citation: https://doi.org/10.5194/gchron-2023-5-AC3
AC4: 'Comment on gchron-2023-5', Guillaume Soulet, 05 Jun 2023

In our reponses to reviewers, we refer to the revised version of our manuscript, that we attach here for them to check our changes. This version will be checked for English by a native speaker in case our manuscript is accepted for publication.
Thanks.

Citation: https://doi.org/10.5194/gchron-2023-5-AC4

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to minor revisions (further review by editor) (15 Jun 2023) by Christine Hatté

Dear Guillaume

Thank you for the revised manuscript and the responses to boths reviewers and to the published comment. You have taken into consideration most of the recommendations and explain why you did not wish to take on board the others. Thanks also for the revised manuscript with line numbers and clear answers that refer to the modified lines in the original article. It helps a lot.

I agree with most of your choices, but would like you to reconsider these few minor points:
* For this type of study, sample size and representativeness are very important. Reducing the sample size also reveals seasonal variability (next point), which is not what is desired here. In the method section, you mention sampling " (30-100 mg) of the outermost layers". Does this sampling cover several seasons? Or is it carried out according to the length of the shell in order to remain within the same hydrological context? Can you clarify this point, or even add photos of the sampling, selecting those representing the extremes of sampling? After all, we're going from simple to triple in the mass sampled.
* the seasonal variability raised by Mr Fontugne may explain certain outliers. Depending on sampling methods, the sample may only record a very short period of life. This is an important and classic bias with the reduction in sampling size. Could you rethink this idea in the discussion, especially at river mouths and in areas of high seasonal variability? Your article would benefit from adding this possibility to the discussion.
* Reviewer #2 also points out that the true measure of 14C is F14C and that the presentation in age 14C is a logarithmic transcription of F14C. I agree. This is also the recommendation of the 14C community (see e.g. Reimer 2004). This is all the more true as 14C ages should be issued rounded (Stuiver and Polach 1977), even if this is not followed by all 14C labs. So, to follow the community's logic while respecting long-standing practices, I recommend presenting results according to SacA-xxx F14C= xxx± xxx (540 ± 23)
* the descriptive list of samples is very important and should not be put off to an appendix. I agree with you. However, reading it is tedious. Can you imagine grouping the information in a table (in landscape mode)? Try it, if it doesn't work then let's stick with the list, perhaps in double columns (if this is allowed in the journal layout)? I'll leave you to consider the various options to get the reader to the end of the article. Attached a proposition of table.

many thanks in advance for the revised version and thanks again for considering Geochronology to publish the results of your research.

all the very best
Christine Hatté

Comments to the Author: PDF

Hide

AR by Guillaume Soulet on behalf of the Authors (21 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (03 Jul 2023) by Christine Hatté

ED: Publish subject to technical corrections (09 Jul 2023) by Philippa Ascough (Editor)

AR by Guillaume Soulet on behalf of the Authors (10 Jul 2023) Manuscript

Short summary

The marine reservoir age (MRA) is the difference between the ¹⁴C age of the ocean and that of the atmosphere at a given time. In geochronology, knowing the local MRA is important to derive accurate calibrated ages for ¹⁴C-dated marine material. However, MRA values for coastal West Africa are scarce. From the ¹⁴C dating of known-age bivalves from museum collections, we calculated MRA values and populated the MRA dataset for coastal West Africa over a latitudinal transect from 33°N to 15°S.